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-- Auxiliary operators --
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-------------------------
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node Even (N: int) returns (B: bool) ;
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let
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  B = (N mod 2 = 0) ; 
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tel
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node ToInt (X: bool) returns (N: int) ;
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let
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  N = if X then 1 else 0 ;
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node Count (X: bool) returns (N: int) ;
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let
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  N = ToInt(X) -> ToInt(X) + pre N ;  
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node Sofar ( X : bool ) returns ( Y : bool ) ;
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let
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 Y = X -> (X and (pre Y)) ;
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node Since ( X, Y : bool ) returns ( Z : bool ) ;
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let
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  Z =  X or (Y and (false -> pre Z)) ;
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node SinceIncl ( X, Y : bool ) returns ( Z : bool ) ;
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let
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  Z =  Y and (X or (false -> pre Z)) ;
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node Increased (N: int) returns (B: bool) ;
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let
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  B = true -> N > pre N ;
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node Stable (N: int) returns (B: bool) ;
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let
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  B = true -> N = pre N ;
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-------------------------------
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-- Stopwatch high-level spec --
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-------------------------------
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-- integrated in the node
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------------------------------
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-- Stopwatch low-level spec --
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------------------------------
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node stopwatch ( toggle, reset : bool ) returns ( count : int );
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(*@
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  -- the watch is activated initially if the toggle button is pressed
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  -- afterwards, it is activated iff 
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  --   it was activated before and the toggle button is unpressed or
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  --   it was not activated before and the toggle button is pressed 
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  var on: bool = toggle ->    (pre on and not toggle) 
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                           or (not pre on and toggle) ;
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  -- we can assume that the two buttons are never pressed together
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  assume not (toggle and reset) ; 
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  -- the elapsed time starts at 1 or 0 depending 
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  -- on whether the watch is activated or not
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  guarantee (on => time = 1) -> true ;
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  guarantee (not on => time = 0) -> true ;
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  -- the elapsed time is always non-negative
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  guarantee time >= 0 ;
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  -- if there is no reset now and there was an even number of counts since the last reset
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  -- then the current elapsed time is the same as the previous one
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  guarantee not reset and Since(reset, Even(Count(toggle))) 
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            => Stable(time) ;
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  -- if there is no reset now and there was an odd number of counts since the last reset
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  -- then the current elapsed time is greater than previous one
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  guarantee not reset and Since(reset, not Even(Count(toggle))) 
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            => Increased(time) ;
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--  guarantee true -> not Even(Count(toggle)) and Count(reset) = 0 => time > pre time ;
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  -- the watch is in resetting mode if the reset button is pressed
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  -- while in that mode, the elapsed time is 0
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  mode resetting ( 
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    require reset ; 
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    ensure time = 0 ; 
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  ) ;
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  -- the watch is in running mode if is activated and 
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  -- the reset button is unpressed.
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  -- while in that mode, the elapsed time increases by 1
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  mode running ( 
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    require on ; 
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    require not reset ; 
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    ensure true -> time = pre time + 1 ; 
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  ) ;
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  -- the watch is in stopped mode if is not activated and 
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  -- the reset button is unpressed
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  -- while in that mode, the elapsed time remains the same
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  mode stopped ( 
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    require not reset ; 
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    require not on ; 
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    ensure true -> time = pre time ; 
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  ) ;
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*)
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  var running : bool;
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let
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  running = (false -> pre running) <> toggle ;
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  count =
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    if reset then 0
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    else if running then 1 -> pre count + 1
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    else 0 -> pre count ;
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